During manufacture of the components making up a rotor module, such as a fan, compressor or turbine blade assembly in a gas turbine, efforts are made to minimise mass imbalances in the individual components. Nevertheless, eccentric mass imbalances will tend to arise in the completed module, for example due to manufacturing tolerances on the blades. Consequently, the module as a whole must undergo a balancing operation in order to prevent subsequent stress and vibration during operation of the module.
The vibration that is of principle concern is generally resonant vibration. This has a maximum amplitude when the speed of rotation of the rotor is at a critical speed (i.e. rotational speed frequency matching a natural frequency of the rotor system). Imbalances within the spool tend to drive the resonant vibration and increase its amplitude. This may cause excessive wear and stress on bearings for the spool, as well as its rotors. The balancing operation thus reduces imbalances and/or balances to compensate for them.
A spool of a gas turbine engine may be made up of plural modules, each treated independently from a balancing point of view (e.g. one module is a compressor assembly and another is a turbine assembly). Each module on the spool is typically balanced in its own right, rather than balance being simply obtained across the whole spool. This allows individual modules to be substituted without a need to re-balance the rest of the spool which would often require disassembly of the entire engine.
In balancing a module the following sources of imbalance are generally considered and compensated for:                1) Unbalance that arises within the module due to geometric errors, such as blades being slightly different weights, or the rotor being slightly radially mal-positioned with respect to the axis of rotation.        2) Unbalance that arises within the module due to the module's geometric error at its interface with another module. In particular, the interface may not be square as a consequence of the sum of miss-alignments of sub-units of the module.        3) Unbalance that arises outside of the module (that is, in the adjacent module) due to the module's geometric error at its interface with that other module.        
To balance each module in its own right, even if the unbalance is of type 3), an unbalance caused by the module must be corrected by making an adjustment within the axial extent of the module on a balancing plane which extends perpendicular to the geometric axis of the module.
A two plane balancing correction is typically carried out by addition or removal of eccentric mass from the module at typically two axially spaced-apart balancing planes. In particular, weight can be added or removed from axially spaced balancing lands, which are usually located at respective ends of the module. This is achieved using a balancing machine, on which the module is rotated and its imbalances are measured. To account for unbalance of type 3), a mass simulator to simulate the (balanced) adjacent module may be used on the balancing machine.
Additionally or alternatively, imbalances can be reduced with particular build techniques such as: component balancing (balancing each component of the module), straight build (eliminating as far as possible the geometric errors that give rise to type 2) and 3) unbalance through careful building up of the sub-units of the module), and blade distribution (arranging the blades of different weights to better balance one another or the components within the module).
A difficulty with the use of balancing lands is that they may be significantly axially spaced from the unbalance that they are compensating for, especially in the case of type 3) unbalance. This axial spacing between the unbalance and its compensating balancing mass creates a bending moment, which may not be detectable at the low rotation speed of the balancing machine. If the bending moment results in flexing at higher rotation speeds, this can create a new imbalance that may drive a resonant vibration.
This is illustrated schematically in FIG. 1, where a module such as a compressor being balanced in a balancing machine is shown attached to a turbine mass simulator. A geometric error (e.g. a non-square joint face) at the module's interface with the mass simulator produces an offset to the mass of the mass simulator. The unbalance generated due to the geometric error is indicated by an upward-pointing cross hatched arrow vector at the mass simulator. The mass simulator allows this unbalance to be detected and corrected by applying corrections indicated by the arrows indicated Fc and Rc at balancing planes of the module. However, a large bending moment, indicated by the graph at the bottom of FIG. 1, is introduced along the rotor by these corrections. The bending moment is not detected by the low speed balancing machine and will cause high vibration in the engine if the module operates at speeds near any flexible rotor modes.
One option is to perform balancing in the balancing machine with mass simulators of different masses to identify the size of the bending moment generated by the simulator. This information can then be used to inform a three location balance correction of the module which can ensure that the bending moment does not excite a flexible mode of the rotor. See, for example, H. Schneider, (2000), Exchangeability of rotor modules—a new balancing procedure for rotors in a flexible state, Seventh International Conference on Vibrations in Rotating Machinery (pp. 101-108), IMechE, where in the example a mass simulator and a “short mandrel” (effectively a zero mass simulator) are interchanged. However, a problem still exists that the time to perform balancing using two mass simulators is very significant. In particular, assembly and disassembly of the modular joint can be time consuming, involving complex joint assembly processes and moving large and easily damaged components around using complex machinery and tooling.
For two mass simulators, all assembly and disassembly steps have to be repeated for the second mass simulator. The entire process can take several working shifts. This problem is exacerbated if the bearing support locations for the mass simulators on the balancing machine are different, such that the balancing machine has to be set up differently for the two mass simulators, as would be the case for the use of a “short mandrel” as described by Schneider ibid.